Amorphous silicon solar cells are being used in increasingly broad applications, ranging from civilian goods such as electronic calculators and watches to electric power supply. In general, amorphous silicon solar cells have a multi-layer structure comprising a base glass plate/transparent conductive film/amorphous silicon film/metal electrode film. Sunlight incident on such a solar cell passes from the base glass plate side through the transparent conductive film and then enters the amorphous silicon film. The base glass plate and the transparent conductive film are hence required to have a high transmittance for good performance.
Such solar cells, when used for electric power supply, need to have a large area exposed to incoming solar radiation. Consequently, these solar cells frequently employ an inexpensive soda-lime glass (alkali-containing glass) produced by the float process as the base glass plate. A thin film of SiO2 (silicon oxide) is often used as a barrier film to prevent migration of alkali ions from the glass into other elements of the multi-layer structure of the film stack. When these solar cells are for use in electric power supply, SnO2 films deposited by CVD are also frequently employed, because these films are relatively inexpensive and highly suitable for mass production and have a higher adhesion strength than SnO2 films deposited by sputtering or vacuum vapor deposition.
Thus, in amorphous silicon solar cells for electric power supply, it is important for the transparent conductive film to have reduced electrical resistance, because these cells have a large panel area. In particular, the transparent conductive film made of SnO2, which is relatively inexpensive, is made to have reduced electrical resistance as a whole by doping the SnO2 with an appropriate impurity and by increasing the thickness of the SnO2 coating.
Glass substrates constituted of a soda-lime glass plate and a two-layer coating formed by successively depositing a continuous alkali barrier film of SiO2 and a transparent conductive film of SnO2 in this order have been subjected to an accelerated test in a high-temperature and high-humidity atmosphere (e.g., 80° C., 100% RH). As a result of such testing, it has been observed that the transparent conductive films having a thickness of 6,000 Å or more developed hair line fractures which inhibit the flow of electrical current flow.